7.4L MPI, M-1 Procharger with 5PSI pulley, gets on plane, runs up to 4600RPM, 5#s of boost, then bogs back to 4200RPM , 2#s of boost and then won't go past 4300RPM. Not under load, the engine runs up to the rev limiter (4800 RPM). In gear it responds all the way up to 4600RPM and then, with in seconds, back to 4200RPM. If I lift the drive up underway the engine will go up to the rev limiter but under full load it falters. I retarded the idle advance a little back from factory spec (8deg). No change. I found the max regulated fuel pressure over 60#s and when I reduced it back to 54#s things got better, (4400RPM). It still goes up on initial plane and then the RPMs and speed drop off. I listened for knock, heard none and I don't think it is going into spark retard due to detonation. Any ideas?

Disconnect knock sensor and try it once, only once!!!

How did you retard the timing back? Did you jump A&B on the harness? If not you haven't changed the timing.

53 - 55 PSI under boost is usually good.

Is you ignition up to the task? Blowers can blow out the flame!! Check Cap, roto and plugs. You may want to tighten up the plug gap if you're not running a HP Ignition system.

Thanks for the response I will try the knock sensor. Should I disconnect at each sensor (one on each side of the block)? I used a merc timing tool to set the timing And I can not remember which two pins it shorts. I went with the next colder plug than factory spec (RV9c) I set the gap@ .035" and the factory setting is .045". I have a stock ignition system. THe fuel pressure seems to fluctuate +/- 10% and when you say boost do you mean WOT?

Blower belt slipping?

I wondered about the blower belt slipping however I am concerned about over tightening the belt. The pro charger manual describes a procedure that makes it way too tight and in fact I Can't even get it to the second de-tent they describe any suggestions on tightening?

anybody have any thoughts on a method for properly tightening the blower belt?

How old is the belt? I know the feeling about getting belt too tight. You might put another turn or 2 on it. Tighten it down & make a run to see if that fixes the problem.

I was told this last weekend about some product that helps belts stick to the pulley & less/no slip. He said they use it at the drags all the time. I will atempt to find out what it is.

I wouldn't do this, That engine has cast pistons and one good ping is all it will take to knock a nice size hole in the piston. You can also bend rods this way.

The ignition is good for 15-PSI as long as you use AC plugs. After many years of super high boost engine experience AC plugs handle boost better that any plug out there at any price. DON'T GET THE AC RAPID FIRES. Those are garbage.

Thanks TurboJack it was the belt. Saw the dust got a new one and installed it with Napa belt dressing. I took the ATI tensioner apart as it would not make it to the second detente like the manual says. It still wont as the spring loaded do hickey it swings on will only travel so far. I took it all the way and the deflection seemed right. I will need to adjust it again as it has broken inn and I saw a 1/2# fall off toward the end of the weekend. Now I know what to look for. By the way the old belt was adjusted the same way and slipped, the old belt looked good TOO.

Glad you got it working.